✨ added return value for emplace (#349)

doc/examples/emplace.cpp

@@ -13,11 +13,18 @@ int main()
     std::cout << null << '\n';
 
     // add values
-    object.emplace("three", 3);
+    auto res1 = object.emplace("three", 3);
     null.emplace("A", "a");
     null.emplace("B", "b");
 
+    // the following call will not add an object, because there is already
+    // a value stored at key "B"
+    auto res2 = null.emplace("B", "c");
+
     // print values
     std::cout << object << '\n';
+    std::cout << *res1.first << " " << std::boolalpha << res1.second << '\n';
+
     std::cout << null << '\n';
+    std::cout << *res2.first << " " << std::boolalpha << res2.second << '\n';
 }